1. Field of the Invention
The present invention generally relates to a mobile communication repeating technology in a cellular mobile communication network. More particularly, the present invention relates to a repeater mounted to a moving object such as a bus, a vehicle, a train, a subway, and a ship and a wireless communication repeating method.
2. Description of the Related Art
In general, a cellular mobile communication network includes a Mobile Switching Center (MSC), a Base Station Controller (BSC), a Base Transceiver Station (BTS), and a Mobile Station (MS). It expands service coverage by installing multiple BTSs. Due to natural or artificial topographical obstacles, a communication shadowing area exists in which an MS cannot receive a signal from a BTS reliably. To relay signals between the BTS and the MS, a wired or wireless repeater is installed in the communication shadowing area. Especially some wireless repeaters are mounted onto moving objects such as mass transportation means provide an active wireless communication service to MSs within the moving objects.
Meanwhile, in a mobile communication service for MSs within a moving object, the MSs move fast between cells and thus handoff occurs very often at short intervals between the MSs of the mobbing object and the BTS of each cell. Therefore, techniques for handling the frequent handoffs or solving the problem are considered significant.
FIG. 1 illustrates a handoff in a mobile communication system. With reference to FIG. 1, a general handoff will be described. Referring to FIG. 1, when an MS 110 moves out of the serving cell area 130 of a serving BTS 120 and enters into another area, for example, another channel, another sector, another BTS, another BSC or another MSC, a handoff function is performed to maintain the communication path of the MS in the mobile communication system.
The handoff functions to switch the MS to new propagation resources and thus to maintain an on-going call, when a signal strength becomes weak during the call due to the movement of the MS. During the handoff, the MS continues to monitor the signal strengths of the serving BTS and neighbor BTSs, compares them, and switches the call to a destination BTS to continue the call.
Although the service area of a single BS covers a few kilometers, it sometimes covers tens of meters in a densely populated area like a metropolitan area. Handoff occurs more frequently when the distance between BTSs is small as in this case.
In the mean time, in a special propagation environment, i.e. when the MS receives a weaker signal from a geographically near BTS than from a relatively remote BTS due to topographical obstacles such as buildings, the MS performs a handoff to the remote BTS offering a good propagation environment, rather than to a neighbor BTS. Once the MS moves out of the special area, it repeats handoff to neighbor BTSs. Particularly when there area many topographical objects near the MS or the MS moves often, a severe multi-path fading results, increasing a handoff occurrence rate.
As a consequence, the call is highly probable to be disconnected in the process of the MS's repeated handoffs and overall system quality is affected.
Soft handoff is a Make Before Break scheme that connects a call after simultaneously catching signals from a plurality of BTSs. Since a plurality of BTSs usually allocate communication channels redundantly to a single MS during handoff, the use efficiency of radio channel resources drops considerably.
Especially for BTSs covering paths of mass transportation vehicles such as buses, the subway, and trains, multiple passengers make calls and move across a plurality of BTSs for a short time. Accordingly, the resulting frequent handoffs decrease the use efficiency of the radio channel resources of the BTSs.